Recently, semiconductor laser diode arrays have been increasingly developed for applications in high-speed optical disk devices or for connections between computers. The following characteristics are required of the semiconductor laser diode arrays: uniform characteristics of individual laser elements, high reliability, and simple fabrication process ensuring good yield. However, since the existing laser diode arrays mostly have a structure in which discrete laser diodes are arranged in parallel on a substrate or the like, the above-described requirements are not satisfied yet.
FIG. 24 is a perspective view illustrating a typical optical disk semiconductor laser. In the figure, reference numeral 201 designates an n type GaAs substrate having opposite front and rear surfaces. An n type AlGaAs first cladding layer 202 is disposed on the front surface of the GaAs substrate 201. An active layer 203 is disposed on the first cladding layer 202. A p type AlGaAs second cladding layer 204 having a stripe-shaped ridge in the center is disposed on the active layer 203. An n type AlGaAs current blocking layer 205 is disposed on the second cladding layer 204, contacting both sides of the stripe-shaped ridge. A p type GaAs first cap layer 206 is disposed on the second cladding layer 204 at the top of the ridge. A p type GaAs second cap layer 209 is disposed on the current blocking layer 205, contacting both sides of the first cap layer 206. A p type GaAs contact layer 211 is disposed on the first and second cap layers 206 and 209. An n side ohmic electrode 215a is disposed on the rear surface of the GaAs substrate 201, and a p side ohmic electrode 215b is disposed on the contact layer 211.
FIGS. 25(a)-25(e) are sectional views illustrating process steps in a method of fabricating the laser structure shown in FIG. 24. In these figures, the same reference numerals as in FIG. 24 designate the same or corresponding parts. Reference numeral 207 designates an SiN mask.
Initially, there are successively grown on the n type GaAs substrate 201 the n type AlGaAs first cladding layer 202, the active layer 203, the p type AlGaAs second cladding layer 204, and the p type GaAs first Gap layer 206 by MOCVD (first crystal growth). Thereafter, an SiN film is deposited on the p type GaAs first cap layer 206 by plasma CVD and patterned in a stripe shape using conventional photolithography and etching techniques, forming an SiN mask 207 (FIG. 25(a)).
In the step of FIG. 25(b), a ridge structure having a reverse mesa shape is formed by wet etching of the first cap layer 206 and the second cladding layer 204 using the SiN mask 207.
Thereafter, the wafer is again set in the MOCVD apparatus, and the n type AlGaAs current blocking layer 205 and the p type GaAs second cap layer 209 are selectively grown on the second cladding layer 204 at the opposite sides of the ridge structure to bury the ridge structure (second crystal growth).
After removal of the SiN mask 207 (FIG. 25(d)), the p type GaAs contact layer 211 is grown over the entire surface of the wafer as shown in FIG. 25(e) (third crystal growth).
To complete the laser structure shown in FIG. 24, the n side ohmic electrode 215a and the p side ohmic electrode 215b are formed on the rear surface of the substrate 201 and on the contact layer 211, respectively, and facets are formed by cleaving.
As an example of a semiconductor laser fabricated in the above-described processing, there is a high-output power TQW (Triple Quantum Well) AlGaAs laser disclosed in, for example, SPIE Vol. 1634, Laser Diode Technology and Applications IV (1992), pp.323 to 328.
In the above-described fabrication method of the optical disk semiconductor laser shown in FIG. 24, since wet etching having poor controllability is employed for the formation of the ridge structure, it is difficult to accurately control the width w that determines the width of the active region and the thickness t of the remaining portion of the second cladding layer 204 that significantly influences the operating characteristics of the laser. That is, the ridge formation process employing wet etching is not suitable for mass production of lasers with uniform characteristics. If a selective etching with an etch stopping layer is employed for the ridge formation, the controllability of the thickness t of the second cladding layer 204 is improved. In this case, however, the introduction of the etch stopping layer may adversely affect the laser characteristics. If the ridge formation process employs HCl gas etching as described in Inst. Phys. Conf. Ser. No. 129, chapter 7, Paper represented at Int. Symp. GaAs and Related Compounds, Karuizawa, 1992, pp. 603-608, the controllability of the ridge width is improved. However, HCl gas etching provides poor controllability of the thickness t of the remaining portion of the second cladding layer 204.
Further, since the control of the ridge width w is difficult as described above, a reduction in the ridge width for a low threshold current cannot be easily achieved. Therefore, a laser that emits a laser beam having an almost circular shape in a section perpendicular to the beam traveling direction cannot be fabricated. That is, it is necessary to provide a completed laser with optical means, such as a lens, for concentrating the laser beam so that an appropriate beam diameter is obtained, resulting in a complicated system.
Further, since the second cladding layer 204 comprises AlGaAs that is easily oxidized, it is oxidized during the ridge etching process, whereby the crystalline quality of the blocking layer 205 regrown on the second cladding layer 204 is significantly degraded.
Further, since the fabrication process includes a lot of steps, i.e., the first epitaxial growth, the wet etching, the second epitaxial growth, the mask removal, and the third epitaxial growth, it is difficult to reduce the cost and improve the yield.
Meanwhile, FIG. 26 is a perspective view illustrating a semiconductor laser with a window structure grown on cleaved facets, that is disclosed in Japanese Journal of Applied Physics, Vol.30, (1991), pp. L904 to L906. In the figure, reference numeral 231 designates a p type GaAs substrate. An n type GaAs current blocking layer 232 is disposed on the substrate 231. A p type Al.sub.0.33 Ga.sub.0.67 As cladding layer 233 is disposed on the current blocking layer 232. A p type Al.sub.0.08 Ga.sub.0.92 As active layer 244 is disposed on the p type cladding layer 233. An n type Al.sub.0.33 Ga.sub.0.67 As cladding layer 235 is disposed on the active layer 244. An n type GaAs contact layer 236 is disposed on the n type cladding layer 235. Reference numeral 237 designates a cleaved (110) facet, and numeral 238 designates an undoped Al.sub.0.4 Ga.sub.0.6 As window layer grown on the facet 237.
The window structure employed in this prior art laser will be described in more detail.
In the AlGaAs high-output power laser, a lot of surface states are produced at the oscillation facets. The surface states cause a reduction in the band gap energy at the facets, compared with the band gap energy in the center of the laser. Therefore, regions adjacent to the facets become light absorption regions with respect to the wavelength of the laser light, and the localized heat generation in the light absorption regions increases with an increase in the light output. Since the band gap energy becomes smaller with the temperature rise, the absorption of the laser light is further encouraged, thereby increasing the temperature at the facets, i.e., so called positive feedback occurs. If the temperature rises sufficiently, localized melting of the semiconductor materials can occur, resulting in catastrophic optical damage (COD) that destroys the laser. COD is a serious problem in AlGaAs series high-output power lasers. In order to reduce the light absorption at the oscillation facets and increase the power level without risk of COD, window layers having a band gap energy higher than a band gap energy equivalent to the oscillation wavelength of the laser are disposed on the oscillation facets of the laser.
A description is given of a fabrication process for the window layer 238 in the laser structure shown in FIG. 26.
Initially, the laser structure is fabricated using conventional wet etching and LPE growth. More specifically, after growth of the n type GaAs current blocking layer 232 on the p type GaAs substrate 231, a stripe-shaped groove is formed in the center of the element so that it penetrates through the current blocking layer 232 and reaches into the substrate 231. Thereafter, the p type Al.sub.0.33 Ga.sub.0.67 As cladding layer 233, the p type Al.sub.0.08 Ga.sub.0.92 As active layer 233, the n type Al.sub.0.33 Ga.sub.0.67 As cladding layer 235, and the n type GaAs contact layer 236 are successively grown on the wafer. After grinding the wafer to a desired thickness, the wafer is cleaved in a plurality of bars each having a width equal to the resonator length of the laser. The resonator length of a typical high-output power laser is 300.about.600 .mu.m. Finally, a material having a band gap energy larger than the band gap energy equivalent to the oscillation wavelength is grown on portions of the bar-shaped wafer corresponding to the resonator facets, preferably by MOCVD.
In this prior art laser, since the laser oscillation wavelength is 830 nm, which is equivalent to 1.49 eV, an undoped Al.sub.0.4 Ga.sub.0.6 As layer having a band gap energy of about 1.93 eV is employed as the window layer 238. After formation of electrodes and coating of the surfaces of the window layers, the bar-shaped wafer is divided into a plurality of laser chips, completing the laser structure shown in FIG. 26. In the prior art literature, Japanese Journal of Applied Physics, Vol.30, L904 to L906, it is reported that the window layer prevents COD and increases the output power and lifetime of the laser.
However, the prior art laser with the window structure shown in FIG. 26 has the following drawbacks.
The laser structure shown in FIG. 26 is fabricated through the complicated process steps as described above. Generally, in fabrication of semiconductor lasers, process steps until the formation of electrodes are carried out on a wafer to secure mass production with high reproducibility. That is, the fabrication method of the prior art laser shown in FIG. 26, in which the window layers are formed on portions corresponding to resonator facets after cleaving of the wafer into the bars each having a width equivalent to the resonator length, provides very poor productivity, so that this method is not useful industrially. Further, when the window layers are grown by MOCVD after the formation of the resonator facets by cleaving, the cleaved facets are easily oxidized and surface states are produced thereon as long as the cleaving is performed in the air. Since the surface states on the facets adversely affect the effect of the window layers, the process steps from the cleaving to the growth of the window layers must be carried out in an inactive gas or in vacuum to avoid the generation of the surface states.
FIG. 27 is a perspective view illustrating a facet non-injection type laser array disclosed in, for example, SPIE Vol.1418, Laser Diode Technology and Applications III (1991), pp. 363.about.371. In the figure, reference numeral 311 designates a p type GaAs substrate. An n type GaAs current blocking layer 312 is disposed on the p type GaAs substrate 311. A p type Al.sub.x Ga.sub.1-x As lower cladding layer 313 is disposed on the current blocking layer 312. An Al.sub.y Ga.sub.1-y As (x&gt;y) active layer 314 is disposed on the lower cladding layer 313. An n type Al.sub.x Ga.sub.1-x As upper cladding layer 315 is disposed on the active layer 314. An n type GaAs cap layer 316 is disposed on the upper cladding layer 315. An n side ohmic electrode 317a and a p side ohmic electrode 317b are disposed on the rear surface of the substrate 311 and on the cap layer 316, respectively.
FIGS. 28(a)-28(d) are perspective views illustrating process steps in a method of fabricating the laser array shown in FIG. 27. This laser array is fabricated through two liquid phase epitaxy (LPE) steps.
Initially, as illustrated in FIG. 28(a), the n type GaAs current blocking layer 312 is grown on the p type GaAs substrate 311 (first LPE growth). Then, as illustrated in FIG. 28(b), a plurality of stripe-shaped grooves are formed in the current blocking layer 312 at intervals of 100 .mu.m by wet etching. Each groove penetrates through the current blocking layer 312 at a region 330 in the center of the resonator but does not penetrate through the current blocking layer 312 at regions 331 adjacent to the facets. When such stripe-shaped grooves are fabricated by wet etching, two etching steps using two different masks are required. The region 330 in which the groove penetrates through the current blocking layer 312 is a current injected region, and the regions 331 in which the groove does not penetrate through the current blocking layer 312 are current blocking regions. The length of each current blocking region is 20 .mu.m, and the channel width of the current injected region is 5.5 .mu.m. The length of the resonator is 600 .mu.m.
Thereafter, as illustrated in FIG. 28(c), the p type Al.sub.x Ga.sub.1-x As lower cladding layer 313, the Al.sub.y Ga.sub.1-y As active layer 314, the n type Al.sub.x Ga.sub.1-x As upper cladding layer 315, and the n type GaAs cap layer 316 are successively grown on the wafer (second LPE growth). Then, as illustrated in FIG. 28(d), the n side ohmic electrode 317a and the p side ohmic electrode 317b are formed by metallizing, and a plurality of grooves 320 having a depth of 20 .mu.m and reaching the substrate 311 are formed by etching, thereby electrically separating the laser elements from each other. Finally, front and rear facets are formed by cleaving, followed by coating of the facets to provide the front and rear facets with a reflectivity of 8% and a reflectivity of 80%, respectively, whereby the laser array shown in FIG. 27 is completed.
In the above-described facet non-injection laser, the density of light at the facets is relatively reduced by the current non-injection regions 331 adjacent to the facets, whereby the COD level is increased. As shown in FIG. 28(b), the current non-injection regions 331 are realized by the grooves that do not penetrate through the current blocking layer 312 in the regions 331. However, it is difficult to produce the grooves using by only one wet etching step. In order to produce the grooves with high reproducibility, two mask patterning and wet etching steps are required, resulting in a very poor productivity.